(Adds remarks from McGuire's attorney) By Kim Palmer CLEVELAND, Jan 16 (Reuters) - A man condemned for a rape and murder convulsed and appeared to struggle for breath during his execution in Ohio on Thursday after a two-drug lethal injection method was used for the first time in the United States, according to media witnesses. Dennis McGuire, 53, who was sentenced to death after his conviction for the 1989 killing of a woman who was seven months pregnant, was the third man executed in the United States this year. McGuire received a combination of the sedative midazolam and pain killer hydromorphone, a mix Ohio created as a substitute option in case it had difficulty obtaining pentobarbital, a drug whose manufacturer has objected to its use in executions. McGuire was pronounced dead at 10:53 a.m. ET (1553 GMT) at a state prison, the Ohio Department of Rehabilitation and Correction said in a statement. A Dayton Daily News reporter, who was present at the execution said; "At 10:29 a.m., his eyes rolled back as if he were going to sleep, and at 10:35 a.m., McGuire, who appeared to be unconscious, was convulsing, gagging and struggling to breathe." Prison spokeswoman, JoEllen Smith declined to comment on the description given by the reporter and other witnesses that McGuire struggled to breathe after the drugs were administered. McGuire's family members witnessing his execution could be seen crying and a family member was heard saying "how could this go on for so long?" the Daily News reported. Afterward, attorney Allen Bohnert, an assistant federal public defender who represented McGuire, urged Ohio Governor John Kasich to declare a moratorium on executions. "The experiment that was this execution has failed," Bohnert said in a telephone interview. Bohnert did not witness the execution, but said reports he received also indicated that McGuire gasped for breath for about 15 minutes after he was injected with the drugs. Lawyers for McGuire had argued in a last minute appeal that the never-before used drug combination would put him at a substantial risk of severe pain and a terrifying inability to breathe before he lost consciousness during the execution. U.S. District Judge Gregory Frost refused to stop the execution, but ordered prison officials to preserve the medication vials, the boxes in which they were packaged and the syringes used for McGuire's execution. McGuire's execution was the third time Ohio has used untested execution drugs. Three botched executions from 2006 to 2009 had resulted in Frost ruling Ohio's lethal injection protocols unconstitutional and ordering a temporary halt to executions. McGuire was convicted of the 1989 rape, kidnapping and murder of Joy Stewart, whose body was found by hikers in woods in western Ohio a day after she had been seen talking with McGuire, according to court records. McGuire initially accused his brother-in-law of killing Stewart, but DNA tests cleared the man and pointed to McGuire. Additional DNA tests years later confirmed McGuire's guilt. Two days before a hearing on his petition for clemency in December, McGuire admitted in a letter to Kasich that he had killed Stewart during a heated argument. Kasich denied him clemency. In a final statement, McGuire thanked Stewart's family for a letter he apparently received from them and said, "To my children, I love you. I'm going to heaven. I'll see you there when you get there," Smith said. Ohio had planned to use the two-drug method in November to execute condemned killer Ronald Phillips, but Kasich stayed his execution to assess whether Phillips' non-vital organs or tissues could be donated after his death. (Editing by David Bailey, Stephen Powell, Sofina Mirza-Reid and Bernard Orr)